Flame treatment or "flame cultivation" is well known in agriculture and horticulture to control weeds and pests that adversely affect arable land. There is a large body of published patent literature concerned with many aspects of this technology, U.S. Pat. No. 2,408,328 setting forth the basic principles of flame cultivation, and thus, reference should be made to this U.S. patent for a detailed explanation of the technique.
Flame cultivation and flaming burners can be incorporated in hand-held torch devices carried by a person for localised or small scale flame cultivation to control weeds and plant pests, particularly in hard to access locations, such as drainage trenches, irrigation ditches and the like, as well as for igniting vegetation for small and large scale clearance of arable land, back-burning operations and the like. Such burners also find use in motorised flame cultivators incorporating a bank or array of burners supported on a boom carried or drawn by a self-propelled agricultural vehicle for large scale surface flame cultivation of arable land.
In the following, the expression torch head and burner will be used synonymously to denote a hollow prismatic or tubular skirt or casing that houses at least one gas delivery jet arrange to emit a jet of combustible fuel (either liquid or gaseous) that is combusted at least partially within the casing so as to generate a flame that is discharged under draft or pressure from an appropriately shaped burner casing mouth. The specific configuration and constructional elements of a flame cultivation and flaming torch heads can vary greatly and is influenced by factors such as the type of application the device is mainly intended for, eg hand-held torches vs torch batteries in an agricultural implement, whether direct flaming or hot combustion gas weeding is to be employed, the fuel supply source, the need for fuel supply regulators and control devices, whether continuous or intermittent high intensity heat is to be applied, the need for a pilot flame or igniters, the required heat energy output rate, burner type (ie combustion of priorly vaporised or vaporising liquid fuel), etc. Accordingly, the large number of patent specifications in this field of technology (international patent classes IPC A01 M 15, F23 D 11, F23 D 14) concern improvements/inventions that are specific to each burner type as well as their specific application.
Agricultural Flame Cultivators
In the field of flame cultivation of large tracts of land, U.S. Pat. No. 3,164,927 (Holloway) discloses a flame cultivator in which a plurality of main burners are supported on a traverse rig or boom mounted on the rear end of a tractor. Two auxiliary burners arranged to direct a gas jet obliquely with respect to the main burner streams (but not intersecting therewith) are mounted on opposite sides outside the prismatic casing of the main burner. All gas delivery jets are supplied with gaseous fuel (butane or propane) from a central supply tank mounted on the tractor via a central supply line and branch lines having regulator valves disposed therein. The gaseous fuel is usually delivered in equal amounts and with equal pressure to the identical fuel delivery jet nozzles, and a mixture of gas and air is created in the burners that is ignited for flame cultivation purposes. It is readily understandable that gas delivery pressure for each jet nozzle can be individually controlled by the associated control valve in the gas delivery lines to each burner to suit varying operational requirements in relation to energy output and flame temperature at each burner. It is also readily apparent that where the burner configuration is modified to use gaseous fuels, an additional vaporiser will be required, as the fuel is conveniently stored in liquid form in a tank, in order to vaporise or convert the liquid fuel into its vapour or gaseous form prior to delivery to the burners. A similar implement is disclosed in U.S. Pat. No. 3,543,436 (Baxter) and U.S. Pat. No. 3,425,407 (Furman et al).
Hand-operated Flame Cultivation and Flaming Torches
Hand-operated torches represent another area of application and are mostly used to destroy plants, e.g. bush, forest undergrowth and the like, in land clearing operations. Such type of torches, referred to as flaming torches, will generally be constructed to generate high intensity flames capable of setting aflame dead wood and live plants alike, whereas with hand-held flame cultivation torches the aim is to destroy unwanted weeds and vermin without setting the crop plants on fire, and thus torch designs are preferred which generate sufficient heat to destroy the cells of young weeds and crop pests without use of high intensity flames that will ignite the vegetation. This aspect is of particular importance in arid zones.
Hand-held torches for agricultural as well as other purposes, e.g. road works, roof taring and the like, generally share a number of common structural features. They have an elongate handle tube or rod which carries at one of its distal ends the torch head which incorporates one (or more) burners. In simple designs, the fuel supply line is connected directly to an LPG hose that is secured at a regulator at the outlet valve of an LPG storage cylinder. More elaborate torches incorporate at the torch itself a deadman shutter (valve) to turn on or off fuel supply to the burners, as well as other regulators and pressure gages in the fuel supply line.
The size of the LPG cylinder of non hand-held torches would usually be determined by the required heat output capacity of the torch burners (as represented by fuel consumption). With hand-held torches though, manoeuvrability of the torch in the field is paramount. For example, small hand-held torches such as those manufactured by Primus and sold under the label "Gardener" and which are mainly intended for the hobby gardener, use small capacity, disposable or refillable LPG cartridges having a self closing connecting valve. Such cartridges can have a capacity of 1 to 3 litres and are attached directly to a fitting at the end of the torch handle. The main limitation as to possible cylinder sizes, however, will generally be their weight. The cylinder must be able to be carried by the torch operator. This is usually accomplished either using a customised back pack or a non-motorised cart. Having regard to restrictions imposed by the location where the torch is to be used and the surrounding terrain, the back pack arrangement is often used in flame cultivation and vegetation torching applications where a larger LPG fuel supply is required, e.g. 5 to 15 litres cylinder capacity.
Smaller hand-held torches incorporate a torch burner arrangement that is relatively lightweight. However, heavy duty, high capacity torch burner heads can have a substantial weight, eg 2 to 4 Kg. Whilst this may not seem much, when mounted at the distal end of a long tubular torch handle of 1.5 m or longer, proper balancing of the torch head by an operator is often difficult and tiring.
Torches with Integral Vaporisers
U.S. Pat. Nos. 3,357,474 and 3,486,497 (both granted to Pivonka) describe torches with combined LPG fuel vaporiser and burner arrangements within a burner casing or head that can equally be integrated in hand-held torches for flame cultivation and igniting vegetation (flaming), as well as large scale flame cultivation implements and machinery.
U.S. Pat. No. 3,357,474 in particular discloses a combined burner-vaporiser torch that uses liquid LPG as fuel source for the burner. LPG is a readily vaporisable liquid fuel (consisting of liquid propane, butane or mixtures thereof, with the possible addition of other hydrocarbons with higher ebullition temperatures). Pivonka specifically discloses the need for a fuel flow control valve in the liquid fuel line running between the liquid fuel storage tank and the vaporiser, the valve being positionable in close vicinity at the rear end of the burner/vaporiser torch; only the forward end portion of the vaporiser and the forward end of the burner shield through which the flames exit the burner will be at a high temperature during torch operation. Due to its constructional arrangement, the rear end of the burner/vaporiser torch remains at temperatures only slightly above ambient temperatures.
When the flow control valve is fully open, LPG is delivered to the vaporiser at a pressure which is essentially dictated by the tank pressure of the LPG storage cylinder. The flow rate of liquid LPG into the vaporiser can be regulated at the flow control valve. In any event, once the liquid LPG exiting the burner nozzles is ignited, because the tubular housing of the vaporiser is in proximity of the flame, the vaporiser chamber will heat up and vaporisation of liquid LPG in the vaporising chamber will take place, thereby lowering the density of the LPG and generating high velocity gas streams therein and consequently also in the burner nozzle chambers. This results in high pressure expelling of fully vaporised fuel through the jet orifices of the burner nozzles once the temperature level within the vaporiser is sufficient to ensure full vaporisation of liquid LPG within the vaporiser chamber, and spitting or flutter in the burner flame is avoided. Pivonka specifically states that the vaporiser-burner device is aimed at providing a high velocity burner flame. Regulation of the flame, and consequently heat energy output by the burners, is carried out solely in the liquid fuel line leading to the vaporiser by way of the conventional flow regulator valve. Such type of arrangement and regulation has inherent weaknesses.
Firstly, and this is a well known phenomenon, regulating flow of a pressurised liquid fuel, which at ambient temperature is gaseous, by means of a flow regulator valve (for example, needle valve) will lead to partial vaporisation of the liquid either in the valve housing itself or downstream of the valve, with the associated take-up of heat by the vaporising resulting in cooling and possible freezing of the fuel lines and fitting. If the flow velocity drop is substantial, the valve itself will tend to frost-up, the ebullition (boiling) temperature of propane fuel being around -42.degree. C. Consequently, liquid fuel supply will be irregular, the vaporiser receiving in some instances liquid and in some instances partially or fully vaporised LPG, depending upon the ambient temperature and pressure induced temperature drop in the fuel supply line between regulator valve and vaporiser. It will be further noted that the torch of Pivonka specifically ensures that the flow regulator in the supply line to the burner-vaporiser torch is not heated by the torch.
A further drawback is the regulating process during steady state burner operation. Regulating the flow of liquid LPG into the vaporiser will only regulate LPG gas burning rate but have little effect on flame velocity over most of the regulator settings. Flame velocity (and burner nozzle gas pressure) regulation would require very fine graduation in a small portion of the wide regulation bandwidth of conventional regulators. In any event, due to the configuration of the vaporiser and the gaseous fuel delivery path to the burner nozzles, notwithstanding liquid LPG flow regulation by means of the control valve, the burner will operate in steady state with high pressure discharge of gaseous fuel and herewith associated high velocity flames which have a long spreading shape. Such high velocity, long spreading flames might be appropriate for some flame cultivation purposes, eg. to penetrate dense or thick plant growth, but is certainly not always necessary nor wanted for flame cultivation or back-burning.
U.S. Pat. No. 3,177,922 (Pardee) discloses a further flame cultivation burner with integrated vaporiser, wherein a flat, box-like burner shield or skirt supports a jet nozzle at a rear closed wall thereof. The nozzle is mounted such as to direct a vaporised jet of gaseous LPG into the inside of the skirt towards the open mouth thereof. The skirt is shaped convergent-divergent to provide a venturi effect in the flow path of the gaseous fuel-air mixture created within the rear section of the shield, combustion air being provided through an air duct opening into the rear section of the shield. The vaporiser itself consists of a separate housing affixed to the top wall of the skirt, the top wall being common to both structures. Liquid LPG is delivered into the vaporising chamber formed within the separate housing by means of a tube which at its open end has a deflector baffle to enhance fluid distribution into the vaporising chamber. A vaporised fuel outlet tube extends from within the vaporising chamber through a wall thereof and is in fluid communication through appropriate rigid lines and fittings with the jet nozzle. All tubing lines extending between the vaporising chamber and the jet nozzle are formed such as to ensure that high pressure gaseous LPG generated within the vaporising chamber (which is effected by heating the additional housing once a flame has been ignited within the burner skirt during normal burner operation) passes unrestricted into the jet nozzle from where it is expelled at high speed and pressure through appropriately sized orifice elements.
The burner/vaporiser torch of Pardee is essentially limited to work at operating pressures (gas pressure at the jet nozzle outlet) dictated by the storage pressure of the liquid LPG in its storage tank, as there is no means of adjusting the operating pressure and hence flame velocity subsequent to vaporisation of the fuel, as pressure drops within the vaporiser and burners are comparatively small. Similar problems exist with the burner device with integrated fuel vaporiser disclosed in U.S. Pat. No. 5,030,086 (Jones)
WO98/01031 (Boral Gas) discloses a vaporiser for use with a flame cultivation device, in which a metering tube is arranged in a hot zone of the vaporiser. The metering tube is arranged for limiting the volume of liquid LPG flow from the LPG storage tank into the vaporisation chamber of the vaporiser. Such vaporiser arrangement disposes with the necessity of fluid LPG flow control by means of a regulator with movable parts and sealing elements that could be subjected to heat corrosion when arranged too close to the hot zone of the vaporiser. Instead, the metering tube ensures a constant supply of liquid LPG into the vaporiser, without the above mentioned icing problems. The fully vaporised gaseous LPG fuel exiting the vaporiser chamber is then regulated in gaseous form using conventional regulator valves for controlling distribution of gaseous fuels to individual burners of a multi-burner implement.
Whilst the basic principle of delivering liquid LPG under supply tank pressure directly into the hot zone within the vaporiser using a fixed metering member with no moving parts and subsequently using conventional regulator valves to control delivery pressure (and amount) of gaseous LPG fuel to the burners of the torch would seem a viable approach in addressing the above mentioned problems associated with the torches of Pivonka and Pardee (ie. trying to regulate the operating pressure of the burners by way of regulating liquid LPG fuel flow into the vaporiser using conventional valves), constructional implementation of such principle would result in vaporiser-burner devices that are expensive because of the need to incorporate gas pressure control valves in the fuel supply lines between vaporiser and burner nozzles, heavy for hand-held implementations and lack the necessary compactness typically required for hand-held torches. Also, typically, gas temperatures of vaporised LPG fuel generated using vaporiser constructions like those of Pivonka, Pardee and Boral Gas, will be in the range of 80.degree.-120.degree. C., this requiring use of regulator valve seals made of heat resistant materials.
Taking into consideration at least some of the problems and drawbacks associated with the above mentioned prior art burner devices with integral fuel vaporisers, the present invention seeks to provide, in at least one preferred embodiment thereof, a burner with integral fuel vaporiser which is able to deliver a low velocity, ground hogging flame for use in flame cultivation that is easy to operate in a safe manner. The burner is to be equally useable in hand-held torches as well as apparatus for flame cultivating rows of crops.
It would also be advantageous for at least a preferred embodiment of the invention to provide a burner device with integral fuel vaporiser that can deliver a high velocity or "booster" flame for use in applications requiring increased heat energy output, in particular as required for flaming.
It would also be advantageous for at least a preferred embodiment of the invention to address balancing problems in particular encountered with hand-held flaming torches that have long torch handles, in a manner which will not or only minimally affect the manoeuvrability of the flaming torch in operation, and provide a hand-held flaming torch that can be safely carried by an operator together with a fuel supply cylinder or canister.